This invention relates generally to a device for measuring the electrochemical activity of a molten slag layer lying on a molten bath of metal, and more particularly, to a sensor for measuring the oxygen potential of oxides present in the slag.
Impurities in pig iron, from which steel is made, consist of carbon, manganese, phosphorus, silicon and sulfur. The concentrations of the aforementioned impurities have a major impact on the characteristics of finished steel. Thus during the refining of molten steel, it is necessary to periodically assess the molten steel for the concentration of the aforementioned impurities.
Conventionally, the concentration of impurities in the molten steel is determined by chemical analysis. The preparation of a sample of the molten steel to conduct the chemical analysis requires taking a sample of the molten steel and allowing the sample to solidify, followed by polishing and washing the sample. The relatively large amount of time required to prepare a sample of molten steel for chemical analysis seriously impacts the ability to correct an undesirable concentration of impurity thus found, particularly in the latter stages of the refining process.
In studies of plant data on slag and steel samples taken at vessel turndown, it has been found, with respect to reactions involving iron oxide in slag, that the slag/steel distribution ratios of phosphorus and sulfur in basic oxygen furnace (BOF) and quick basic oxygen (Q-BOP) steel making processes are scattered about the slag-steel equilibrium values. Further studies indicate consistent inter-relationships between the concentrations of FeO, SiO2 and CaO in the slag at the end of the oxygen blow and the concentrations of sulfur and phosphorus in the molten steel, in both the BOF and Q-BOP processes. Thus, by rapidly determining the concentration of FeO, SiO2 and CaO in the slag, in real time, near the end of the oxygen blow, corrective steps can be taken by appropriate flux additions to further dephosphorization and desulfurization of steel during the production of the steel.
The activity aox. of a molten oxide in molten slag is defined by the expression:                                           a                          ox              .                                =                                    (                                                                    p                                          o                      2                                                        ⁢                                      ox                    .                                                                                        p                                          o                      2                                        o                                    ⁢                                      ox                    .                                                              )                                      1              /              2                                      ,                            (        1        )            
where po2 ox. is the oxygen partial pressure of the slag in equilibrium with the actual liquid oxide in the slag and po2o ox. is the oxygen partial pressure of the slag in equilibrium with pure liquid oxide.
Substantial amounts of experimental data have been collected over the years that relates the activity of FeO, SiO2 and CaO in slag to the concentrations of the aforementioned metal oxides in the slag. Consequently, by measuring the activities of FeO, SiO2 and CaO in slag, the concentrations of FeO, SiO2 and CaO in the slag can be determined.
When an electrochemical sensor comprising two dissimilar (metal alloy+metal oxide) electrodes is immersed in molten slag or glass, there is created a difference in oxygen potentials at the slag-electrode interface between the two electrodes. The difference in oxygen potentials establishes an open circuit cell electromotive force (emf) by galvanic action that is a measure of the particular metal oxide activity of the slag. The open circuit emf of the galvanic cell thus formed by the two electrodes in contact with the molten slag can be expressed by the following well known thermodynamic relation:                                                         10.08              ·              E                        ⁢                          xe2x80x83                        ⁢                          (              mv              )                                T                =                                            log              ⁡                              (                                                      p                                          o                      2                                                        ⁢                                      ox                    .                                                  )                                                    1              /              2                                -                                    log              ⁡                              (                                                      p                                          o                      2                                                        ⁢                                      ref                    .                                                  )                                                    1              /              2                                                          (        2        )            
where E (mv) is the sensor open circuit emf in millivolts, T is the absolute temperature in xc2x0 K and po2 ox. and po2 ref. are the equilibrium oxygen partial pressures at the oxide and reference electrodes respectively. Combining equations (1) and (2) yields the following expression that relates the oxygen partial pressures at the oxide and reference electrodes to the activity of the oxide in the slag:                               log          ⁢                      xe2x80x83                    ⁢                      a                          ox              .                                      =                                            10.08              ·              E                        T                    +                                    log              ⁡                              (                                                                            p                                              o                        2                                                              ⁢                                          ref                      .                                                                                                  p                                              o                        2                                            o                                        ⁢                                          ox                      .                                                                      )                                                    1              /              2                                                          (        3        )            
The values of po2o ox. and po2 ref. are constants for a particular electrode couple. The thermochemical data used for calculating the constants po2o ox. and po2 ref. for the electrode couples of the preferred embodiments are drawn from Baron, I., and Knacke, O., xe2x80x9cThermochemical Properties of Inorganic Substancesxe2x80x9d, Springer-Verlag, Berlin, 1973; Baron, I., Knacke, O. and Kubaschewski, O., xe2x80x9cThermochemical Properties of Inorganic Substances, Supplementxe2x80x9d, Springer-Verlag, Berlin, 1977; Turkdogan, E. T., Ironmaking and Steelmaking, 1993, 20(6), 469; and Hultgren, R., Desai, P. D., Hawkins, D. T., Gleiser, M. and Kelley, K. K., xe2x80x9cSelected Values of Thermodynamic properties of Binary Alloysxe2x80x9d, ASTM, 1973.
An electrochemical sensor having a galvanic action for measuring the activity of iron oxide in various types of molten slags in equilibrium with iron is disclosed in U.S. Pat. No. 4,657,641. The patent discloses a reference electrode 4 of a powder mixture of (Mo+MoO2) or (Cr+Cr2O3) incased in a stabilized ZrO2 thimble electrolyte 3, and a Mo lead line 6 immersed in the molten metal to complete the electric circuit (see particularly FIGS. 1 and 10). With this type of sensor, i.e. having a solid electrolyte interposed between the slag and the reference electrode, the value of the po2 ref at the slag-electrolyte interface is ill defined because it is a function of both the oxygen transfer rate across the electrolyte wall (a diffusion process) to or from the reference electrode and a redox reaction at the slag-electrolyte interface.
Another slag-FeO sensor for measuring the in-situ oxygen activity of slag has been developed by Heraeus Electro-Nite. The sensor, known as Quick-Slag(copyright) comprises a Cr+Cr2O3 reference electrode encased in a ZrO2+MgO solid electrolyte. In use, the sensor is immersed through the slag layer into the molten steel bath, a small amount of slag being collected on the solid electrolyte resulting in a slag/metal interface on the sensor. The sensor has been extensively tested, as reported by Meszaros, G. A. et al., xe2x80x9cImplementation of a Ladle Slag Oxygen Activity Sensor to Optimize Ladle Slag Practices at the U.S. Steel Mon Valley Worksxe2x80x9d, Iron and Steelmaker, 1997, 24(7), 33. The test results of the sensor show the sensor emf output increasing as FeO content of the slag increases, although the measurements do not register the theoretical relationship between the FeO activity and the sensor emf readings.
While the above described sensors are available for measuring the activity of FeO in slag, there is a need in the art for a sensor which more accurately determines the activity of FeO in slag, and preferably, is also able to simultaneously measure the activity of more than one kind of oxide present in the slag. By recognizing that the molten slag itself is an electrolyte, the need for a solid electrolyte is eliminated in constructing an electrochemical sensor. Accordingly, by the eliminating the uncertainty in the value of po2 ref. caused by the diffusion process through the solid electrolyte, the accuracy of the sensor is increased.
Briefly stated the present invention provides an electrochemical sensor for insertion into slag for determining an activity of a metal oxide in the slag. The sensor includes a reference electrode comprising a mixture of a metal alloy and a metal oxide, the metal alloy and the metal oxide each being exposed on a surface of the reference electrode. The sensor also includes at least one oxide electrode comprising a mixture of a metal alloy and a metal oxide different from the mixture of the reference electrode, the metal alloy and the metal oxide of the oxide electrode each being exposed on a surface of the oxide electrode. The sensor further includes a refractory housing holding the reference electrode and the at least one oxide electrode wherein the reference electrode and the at least one oxide electrode extend from the refractory housing such that the surface of the reference electrode and the surface of the oxide electrode are each in direct contact with the slag when the sensor is inserted into the slag.
The present invention also provides a sensor having an electrochemical reaction when inserted into molten slag for determining an activity of a metal oxide in the slag. The sensor includes a reference electrode comprising a mixture of a metal alloy and a metal oxide; at least one oxide electrode comprising a mixture of a metal alloy and a metal oxide different from the mixture of the reference electrode; and a refractory housing holding the reference electrode and the oxide electrode in direct contact with the slag when the sensor is inserted into the slag, wherein the electrochemical reaction of the sensor relies on the electrolytic properties of the molten slag.
The present invention further provides a method for determining the activity of at least one metallic oxide in slag. The method comprises the steps of inserting an electrochemical sensor having a reference electrode and at least one oxide electrode into the slag such that the reference electrode and each oxide electrode are in direct contact with the slag and not in contact with molten metal beneath the slag; measuring a temperature of the slag; and measuring an electromotive force between the reference electrode and each oxide electrode.